Communication Terminal Apparatus And Wireless Transmission Method

ABSTRACT

A communication terminal apparatus that exhibits a shorter time period required until a start of communication and causes the throughput in the wireless communication system to be less reduced. In this apparatus, when the number, N, of retransmissions notified by a response determining part ( 107 ) is zero, a subchannel-to-be-used selecting part ( 108 ) selects a subchannel, among others, which exhibits the highest order one of the reception qualities of the subchannels notified by a reception quality determining part ( 106 ), and then notifies the selected subchannel to a subchannel allocating part ( 113 ). When the notified number, N, of retransmissions is one or more, the response determining part ( 107 ) selects a subchannel which exhibits the N-th lower order reception quality than the highest order reception quality, based on the reception qualities of the subchannels notified by the reception quality determining part ( 106 ) for a pilot signal received after a transmission of the latest access request signal, and then notifies the selected subchannel to the subchannel allocating part ( 113 ).

TECHNICAL FIELD

The present invention relates to a communication terminal apparatus thatperforms radio communication with a base station apparatus and the like,and a radio transmission method.

BACKGROUND ART

Conventionally, in a radio communication system, when a communicationterminal apparatus such as a mobile telephone starts radio communicationsuch as packet transmission, the communication terminal apparatusreceives a pilot signal periodically transmitted from the base stationapparatus, performs open-loop transmission power control (OL-TPC) on anaccess request signal based on reception quality of the pilot signal,and transmits the access request signal to the base station apparatususing a random access channel (RACH), and when receiving the accessrequest signal, the base station apparatus transmits an accesspermission signal to the communication terminal apparatus using aforward access channel (FACH). Then, after receiving the accesspermission signal, the communication terminal apparatus transmitstransmission data to the base station apparatus using an uplink datachannel.

In such a radio communication system, when a plurality of communicationterminal apparatuses transmit access request signals concurrently usingthe same resource on the RACH, collision of access request signalsoccurs on the propagation path, and therefore the base station apparatuscannot receive the access request signals, and as a result, does nottransmit access permission signals to the communication terminalapparatuses.

Therefore, a technique is developed that, when an access permissionsignal is not transmitted within a predetermined response waiting periodafter transmitting the access request signal, the communication terminalapparatus retransmits the access request signal after a lapse ofback-off time randomly set using a timing at which the access requestsignal was previously transmitted as a reference (for example, seePatent Document 1).

FIG. 1 schematically shows the technique as described in PatentDocument 1. In FIG. 1, communication terminal apparatuses land 2 firsttransmit access request signals concurrently on the same sub-channel ofthe RACH, and therefore, the base station apparatus cannot receiveeither of the access request signals. Then, an access permission signalis not transmitted from the base station apparatus within apredetermined response waiting period after the first access requestsignal is transmitted, and therefore both of communication terminalapparatuses 1 and 2 retransmit access request signals to the basestation apparatus after a lapse of randomly set back-off time 1 orback-off time 2, respectively.

Thus, in the technique as described in Patent Document 1, when collisionof access request signals occurs, by randomly setting the back-offtimes, the probability of occurrence of collision of retransmittedaccess request signals is reduced.

Patent Document 1: Japanese Patent Application Laid-Open No. 2000-308148

DISCLOSURE OF INVENTION Problems to be Solved by the Invention

However, in the technique as described in Patent Document 1, the timeuntil retransmission of the access request signal is naturally longerthan the response waiting period, and therefore, when collision ofaccess request signals occurs, there is a problem that the time requiredfor the communication terminal apparatus to start communication becomeslong. Further, in the technique as described in Patent Document 1, inthe radio communication system having such communication terminalapparatuses as components, the time required to start communicationbecomes long, and thereby a problem arises that the throughput tends todecrease.

It is therefore an object of the present invention to provide acommunication terminal apparatus and radio communication method forreducing the time required to start communication and suppressingreduction in throughput in a radio communication system.

MEANS FOR SOLVING THE PROBLEM

A communication terminal apparatus according to the present inventionadopts a configuration provided with: a sub-channel selector thatselects a sub-channel to be used in transmission of a random accesssignal from a group of sub-channels defined by a class of frequency andselects the sub-channel different from previously used sub-channelwhenever retransmitting the random access signal; and a transmitter thattransmits the random access signal using the selected sub-channel.

ADVANTAGEOUS EFFECT OF THE INVENTION

According to the communication terminal apparatus according to thepresent invention, an access request signal is transmitted orretransmitted using a sub-channel selected from a group of sub-channelsdefined by a class of frequency, so that it is possible to reduce theprobability of occurrence of collision of access request signals. As aresult, according to the communication terminal apparatus according tothe present invention, the number of retransmissions of the accessrequest signal can be reduced, so that it is possible to startcommunication in a short time, and suppress reduction in throughput inthe radio communication system. Further, according to the communicationterminal apparatus according to the present invention, it is possible toimprove the communication quality of packet communication havingexacting delay requirement, such as speech communication and videotransmission.

Moreover, according to the communication terminal apparatus according tothe present invention, the scheme of selecting a sub-channel can bechanged in the case of first transmitting an access request signal andin the case of retransmitting the access request signal after collisionof access request signals occurs so that it is possible to furtherreduce the probability of occurrence of collision of access requestsignals in the case of retransmitting the access request signal.

Further, according to the communication terminal apparatus according tothe present invention, when an access request signal is transmittedusing a sub-channel with good reception quality of a pilot signal, it ispossible to suppress the transmission power of the access requestsignal. As a result, in the communication terminal apparatus accordingto the present invention, power consumption can be suppressed, so thatit is possible to extend the communication time when the battery isused. Furthermore, according to the communication terminal apparatusaccording to the present invention, when the communication terminalapparatus is, for example, a mobile telephone located near the celledge, it is possible to prevent an access request signal transmittedfrom the communication terminal apparatus from being an interferingsignal in adjacent other cells and reducing throughput in the othercells.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 schematically shows the operation of a conventional communicationterminal apparatus upon start of communication;

FIG. 2 is a block diagram illustrating a configuration of acommunication terminal apparatus according to Embodiment 1 of thepresent invention;

FIG. 3 schematically shows the operation of the communication terminalapparatus upon start of communication according to Embodiment 1 of thepresent invention;

FIG. 4 is a block diagram illustrating a configuration of acommunication terminal apparatus according to Embodiment 2 of thepresent invention;

FIG. 5 schematically shows the operation of the communication terminalapparatus upon start of communication according to Embodiment 2 of thepresent invention;

FIG. 6 schematically shows the operation of the communication terminalapparatus upon start of communication according to Embodiment 2 of thepresent invention;

FIG. 7 is a block diagram illustrating a configuration of acommunication terminal apparatus according to Embodiment 3 of thepresent invention;

FIG. 8 schematically shows the operation of the communication terminalapparatus upon start of communication according to Embodiment 3 of thepresent invention;

FIG. 9 is a block diagram illustrating a configuration of acommunication terminal apparatus according to Embodiment 4 of thepresent invention;

FIG. 10 schematically shows the operation of the communication terminalapparatus upon start of communication according to Embodiment 4 of thepresent invention;

FIG. 11 is a block diagram illustrating a configuration of acommunication terminal apparatus according to Embodiment 5 of thepresent invention;

FIG. 12 schematically shows the operation of the communication terminalapparatus upon start of communication according to Embodiment 5 of thepresent invention;

FIG. 13 is flowchart illustrating a method of selecting a sub-channel ina using sub-channel selecting section as shown in FIG. 11;

FIG. 14 is a block diagram illustrating a configuration of acommunication terminal apparatus according to Embodiment 6 of thepresent invention;

FIG. 15 schematically shows the operation of the communication terminalapparatus upon start of communication according to Embodiment 6 of thepresent invention; and

FIG. 16 is a flowchart illustrating the method of selecting asub-channel.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described in detail belowwith reference to accompanying drawings as appropriate. Each offollowing Embodiments describes the case, as an example, where aplurality of communication terminal apparatuses transmit OFDM(Orthogonal Frequency Division Multiplexing) signals to a base stationapparatus in a radio communication system of a cellular scheme. Inaddition, in the Embodiments, components with the same function areassigned the same reference numerals without further explanations.

Embodiment 1

Embodiment 1 of the present invention describes the case where an OFDMsignal is transmitted or received in a time division duplex (TDD)scheme, as an example. FIG. 2 is a block diagram illustrating aconfiguration of communication terminal apparatus 100 according toEmbodiment 1 of the present invention. Communication terminal apparatus100 has radio reception section 101, FFT (Fast Fourier Transform)section 102, channel dividing section 103, demodulation section 104,decoding section 105, reception quality measuring section 106, responsedetermining section 107, using sub-channel selecting section 108, codingsections 111-1 and 111-2, modulation sections 112-1 and 112-2,sub-channel assigning section 113, transmission power control section114, IFFT (Inverse Fast Fourier Transform) section 115, radiotransmission section 116 and antenna element 117.

Radio reception section 101 receives a pilot signal, access permissionsignal and the like transmitted by radio from a base station apparatusvia antenna element 117, performs predetermined reception processingsuch as frequency conversion and analog/digital conversion on thereceived signal, and inputs the received signal subjected to thereception processing to FFT section 102.

FFT section 102 converts the received signal inputted from radioreception section 101 from a serial signal into a parallel signal,performs FFT processing on the parallel signal, further converts theresult into a serial signal, thereby converts a symbol arrangement inthe frequency axis direction in the received signal into a symbolarrangement in the time axis direction, and inputs the received signalsubjected to symbol arrangement conversion to channel dividing section103.

Channel dividing section 103 determines a channel of the received signalinputted from FFT section 102, and thereby determines whether or not thereceived signal is a pilot signal. Then, when the received signal is thepilot signal, channel dividing section 103 inputs the received signal toreception quality measuring section 106. Meanwhile, when the receivedsignal is not the pilot signal, channel dividing section 103 inputs thereceived signal to demodulation section 104.

Demodulation section 104 demodulates the received signal inputted fromchannel dividing section 103 with a predetermined scheme, and inputs thedemodulated received signal to decoding section 105.

Decoding section 105 decodes the received signal inputted fromdemodulation section 104 with a predetermined scheme, generatesreception data, and inputs the generated reception data to responsedetermining section 107, and a control section and the like not shown.

Reception quality measuring section 106 measures reception quality, forexample, Signal-to-Interference power Ratio (SIR) or reception powerlevel of the pilot signal inputted from channel dividing section 103 foreach sub-carrier group—for each sub-channel defined by a class offrequency in an OFDM signal—and reports the measurement result to usingsub-channel selecting section 108.

Response determining section 107 detects a timing at which an accessrequest signal is transmitted from radio transmission section 116,performs error detection by Cyclic Redundancy Checking (CRC) on thereception data inputted from decoding section 105, and therebydetermines whether an access permission signal is transmitted from thebase station apparatus within a predetermined response waiting periodfrom the transmission timing of the access request signal. Further, whenthe access request signal is first transmitted to the base stationapparatus, response determining section 107 reports to using sub-channelselecting section 108 that the number of retransmissions of the accessrequest signal is zero. Further, when determining that an accesspermission signal is not transmitted from the base station apparatuswithin the predetermined response waiting period from the transmissiontiming of the first access request signal, response determining section107 counts the number of retransmissions of the access request signal asone, and reports to using sub-channel selecting section 108 that thenumber of retransmissions of next transmitted access request signal isone after a lapse of the response waiting period. Then, responsedetermining section 107 increases the number of retransmissions of theaccess request signal by one whenever the response waiting period haslapsed for each access request signal until the base station apparatustransmits an access permission signal in response to the sequentiallytransmitted access request signals, and reports the counted number ofretransmissions to using sub-channel selecting section 108. Further,when determining that the access permission signal is transmitted fromthe base station apparatus within the response waiting period of theaccess request signal, response determining section 107 reports thedetermination result to using sub-channel selecting section 108.

When the number of retransmissions reported from response determiningsection 107 is zero, using sub-channel selecting section 108 selects asub-channel with the highest reception quality reported for eachsub-channel from reception quality measuring section 106, and reportsthe selected sub-channel to sub-channel assigning section 113. Further,when the number of retransmissions reported from response determiningsection 107 is one or more, using sub-channel selecting section 108selects a sub-channel with lower reception quality corresponding to thereported number of retransmissions from the highest reception qualitybased on the reception quality reported for each sub-channel fromreception quality measuring section 106 on the pilot signal that isreceived after the access request signal is last transmitted, andreports the selected sub-channel to sub-channel assigning section 113.Further, using sub-channel selecting section 108 reports a transmissionpower level associated with the reception quality of thus selectedsub-channel to transmission power control section 114. In addition, thereception quality of the sub-channel reported from reception qualitymeasuring section 106 is associated with the transmission power level ofthe access request signal in advance, so that the transmission powerlevel of the access request signal decreases in accordance with anincrease in the reported reception quality. Further, when responsedetermining section 107 reports the determination result that the accesspermission signal is transmitted, using sub-channel selecting section108 reports the determination result to channel assigning section 113,and reports the transmission power level associated with the receptionquality to transmission power control section 114 based on the receptionquality for each sub-channel on the pilot signal subsequently reportedfrom reception quality measuring section 106.

Coding section 111-1 encodes the access request signal inputted from thecontrol section and the like not shown with a predetermined scheme, andinputs the coded access request signal to modulation section 112-1.Further, coding section 111-2 encodes transmission data inputted fromthe control section and the like not shown with a predetermined scheme,generates a transmission signal, and inputs the generated transmissionsignal to modulation section 112-2.

Modulation section 112-1 modulates the access request signal inputtedfrom coding section 111-1 with a predetermined scheme, and inputs themodulated access request signal to sub-channel assigning section 113.Further, modulation section 112-2 modulates the transmission signalinputted from coding section 111-2 with a predetermined scheme, andinputs the modulated transmission signal to sub-channel assigningsection 113.

Sub-channel assigning section 113 assigns the sub-channel—sub-carriergroup—or frequency reported from using sub-channel selecting section 108to the modulated access request signal inputted from modulation section112-1, and inputs the access request signal to which the sub-channel isassigned to transmission power control section 114. Further, when usingsub-channel selecting section 108 reports the determination result thatthe access permission signal is transmitted, sub-channel assigningsection 113 assigns the transmission signal inputted from modulationsection 112-2 to a sub-channel instructed from the control section andthe like not shown or a predetermined sub-channel and inputs the resultto transmission power control section 114.

Transmission power control section 114 amplifies the access requestsignal or transmission signal inputted from sub-channel assigningsection 113 to a transmission power level reported from usingsub-channel selecting section 108, and inputs the amplified accessrequest signal or transmission signal to IFFT section 115.

IFFT section 115 converts the access request signal or transmissionsignal inputted from transmission power control section 114 into aparallel signal, performs inverse fast Fourier transform on the parallelsignal, thereby converts a symbol arrangement in the time axis directioninto a symbol arrangement in the frequency axis direction, furtherconverts into a serial signal, generates an OFDM signal, and inputs thegenerated OFDM signal to radio transmission section 116.

Radio transmission section 116 performs predetermined transmissionprocessing such as digital/analog conversion and frequency conversion onthe OFDM signal that is the access request signal or transmission signalinputted from IFFT section 115, and radio transmits the OFDM signal tothe base station apparatus via antenna element 117.

The operation of communication terminal apparatus 100 will be describedbelow with reference to FIG. 3.

In FIG. 3, it is assumed that communication terminal apparatuses 100-1and 100-2 concurrently transmit access request signals to the same basestation apparatus, and that the reception SIR is measured for eachsub-channel in the pilot signal in reception quality measuring section106 of each of communication terminal apparatuses 100-1 and 100-2.Further, it is assumed in FIG. 3 that in communication terminalapparatus 100-1, with respect to the reception SIR for each sub-channelin the pilot signal, due to the effect of frequency selective fading,sub-channel #1 is 6 dB, sub-channel #2 is −4 dB, sub-channel #3 is 10dB, sub-channel #4 is 15 dB, sub-channel #5 is 2 dB, and sub-channel #6is 5 dB. Similarly, it is assumed that in communication terminalapparatus 100-2, with respect to the reception SIR for each sub-channelin the pilot signal, sub-channel #1 is 8 dB, sub-channel #2 is −3 dB,sub-channel #3 is 5 dB, sub-channel #4 is 18 dB, sub-channel #5 is 12dB, and sub-channel #6 is 5 dB. It is further assumed in FIG. 3 that thereception SIR of the first received pilot signal is the same as thereception SIR of the subsequently received pilot signal in communicationterminal apparatuses 100-1 and 100-2.

In FIG. 3, in first transmitting access request signals, each ofcommunication terminal apparatuses 100-1 and 100-2 selects a sub-channelwith the highest reception SIR in the pilot signal, and thus selects thesame sub-channel #4. Therefore, neither of the access request signalsfirst transmitted from communication terminal apparatuses 100-1 and100-2 can be received in the base station apparatus. As a result,neither of communication terminal apparatuses 100-1 and 100-2 canreceive access permission signals in response to the first accessrequest signals within a response waiting period, and therefore, uponreception of a next pilot signal, access request signals areretransmitted. In retransmission of the access request signals, each ofcommunication terminal apparatuses 100-1 and 100-2 selects a sub-channelwith the lower reception SIR of the pilot signal corresponding to thenumber of retransmissions from the highest reception quality. Therefore,communication terminal apparatus 100-1 selects sub-channel #3, andcommunication terminal apparatus 100-2 selects sub-channel #5.Accordingly, the base station apparatus receives both the access requestsignals retransmitted from communication terminal apparatuses 100-1 and100-2.

Thus, according to communication terminal apparatus 100 according tothis Embodiment, a different sub-channel is selected according to thenumber of retransmissions, and an access request signal is transmittedor retransmitted using the selected sub-channel, so that it is possibleto decrease the probability of occurrence of collision of access requestsignals. As a result, according to communication terminal apparatus 100according to the present invention, it is not necessary to set a randomback-off time upon retransmission of the access request signal, and itis possible to reduce the number of retransmissions of the accessrequest signal, so that it is possible to start the communication in ashort time, suppress reduction in throughput in the radio communicationsystem, and improve the communication quality in packet communicationhaving exacting delay requirement, such as speech communication andvideo transmission.

Further, according to communication terminal apparatus 100 according tothis Embodiment, OL-TPC is performed on the access request signal andtransmission signal based on the reception SIR of the pilot signal, sothat, for example, when communication terminal apparatus 100 is a mobiletelephone and located near the cell edge, it is possible to prevent anaccess request signal and transmission signal transmitted fromcommunication terminal apparatus 100 from being an interfering signal inadjacent other cells and reducing the throughput. Furthermore, accordingto communication terminal apparatus 100 according to this Embodiment, asub-channel for use in transmission of an access request signal isselected in descending order of the reception SIR according to thenumber of retransmissions, and therefore the transmission power of theaccess request signal becomes larger starting with the minimum value, sothat it is possible to further reduce the interference imposed onadjacent other cells, and as a result, effectively prevent reduction inthroughput due to interference in other cells.

Moreover, according to communication terminal apparatus 100 according tothis Embodiment, the scheme of selecting a sub-channel is changed in thecase of first transmitting an access request signal and in the case ofretransmitting the access request signal after collision of accessrequest signals occurs, so that it is possible to further decrease theprobability of occurrence of collision of access request signals in thecase of retransmitting the access request signal.

In addition, in this Embodiment the case has been described where usingsub-channel selecting section 108 changes the scheme of selecting asub-channel according to the number of retransmissions of the accessrequest signal reported from response determining section 107, but thepresent invention is not limited to this case. For example, usingsub-channel selecting section 108 may continuously select a sub-channelwith the highest reception quality even when the number ofretransmissions of the access request signal reported from responsedetermining section 107 is one or more. By this means, it is possible toreduce the load of signal processing required for selection ofsub-channel on using sub-channel selecting section 108. In this regard,when such selection is made in the case where the fading fluctuation isfast on the propagation path, the probability of occurrence of collisionof access request signals is further reduced.

Embodiment 2

FIG. 4 is a block diagram illustrating a configuration of communicationterminal apparatus 300 according to Embodiment 2 of the presentinvention. Communication terminal apparatus 300 has using sub-channelselecting section 308 instead of using sub-channel selecting section108, and threshold setting section 321 between response determiningsection 107 and using sub-channel selecting section 308 in communicationterminal apparatus 100 according to Embodiment 1.

Based on the reception quality for each sub-channel reported fromreception quality measuring section 106, using sub-channel selectingsection 308 randomly selects one from a group of sub-channels with thereception quality more than or equal to a threshold reported fromthreshold setting section 321, and reports the selected sub-channel tosub-channel assigning section 113. Further, using sub-channel selectingsection 308 reports a transmission power level associated with thereception quality of the selected sub-channel to transmission powercontrol section 114.

According to the number of retransmissions, n, reported from responsedetermining section 107, threshold setting section 321 reports athreshold calculated, for example, from an equation of“threshold=αdB−n×βdB” where αdB is an initial value, and βdB is avariable coefficient, to using sub-channel selecting section 309. Inaddition, the initial value is set at 5 dB (αdB=5 dB) and the variablecoefficient is set at 2 dB (βdB=2 dB) in the following descriptions.

The operation of communication terminal apparatus 300 will be describedbelow with reference to FIGS. 5 and 6.

FIG. 5 shows the reception SIR for each sub-channel in the pilot signalmeasured in communication terminal apparatus 300. In addition, thereception SIR for each sub-channel as shown in FIG. 5 is the same asthat in FIG. 3.

When an access request signal is first transmitted, it is reported thatthe number of retransmissions is zero from response determinationsection 107, and therefore threshold setting section 321 reports thethreshold=5 dB-0×2 dB=5 dB to using sub-channel selecting section 308.Then, using sub-channel selecting section 308 randomly selects one fromsub-channel #1 of 6 dB, sub-channel #3 of 10 dB, sub-channel #4 of 15 dBand sub-channel #6 of 5 dB which have the reception SIR more than orequal to 5 dB, reports the selected sub-channel to channel assigningsection 113, and further reports a transmission power level associatedwith the reception SIR to transmission power control section 114.

Similarly, threshold setting section 321 reports the threshold=5 dB−1×2dB=3 dB to using sub-channel selecting section 308 when it is reportedthat the number of retransmissions is one from response determiningsection 107, and reports the threshold=5 dB−2×2 dB=1 dB to usingsub-channel selecting section 308 when it is reported that the number ofretransmissions is two from response determining section 107. Then, asshown in FIGS. 5(B) and 5(C), using sub-channel selecting section 308randomly selects one from sub-channels with the reception SIR more thanor equal to 3 dB or 1 dB, respectively, reports the selected sub-channelto channel assigning section 113, and further reports a transmissionpower level associated with the reception SIR to transmission powercontrol section 114.

FIG. 6 shows an example of a mode where the reception SIR measured foreach sub-channel in the pilot signal in communication terminalapparatuses 300-1 and 300-2 is as shown in FIG. 5, and communicationterminal apparatuses 300-1 and 300-2 concurrently transmit andretransmit access request signals to the same base station apparatus. Inthe example as shown in FIG. 6, both of communication terminalapparatuses 300-1 and 300-2 select sub-channel #4 when the number ofretransmissions is zero corresponding to FIG. 5(A), and selectsub-channel #3 when the number of retransmissions is one correspondingto FIG. 5(B), and in theses cases, the access request signals cannot bereceived in the base station apparatus. Then, when the number ofretransmissions is two corresponding to FIG. 5(C), communicationterminal apparatus 300-1 selects sub-channel #4, and communicationterminal apparatus 300-2 selects sub-channel #1, and therefore accessrequest signals of the apparatuses 300-1 and 300-2 are both received inthe base station apparatus for the first time.

In this Embodiment, when the threshold used in using sub-channelselecting section 308 is made smaller, the probability increases thatthe access request signal is transmitted or retransmitted using asub-channel with a low reception SIR, and therefore an error rate of theaccess request signal increases in the base station apparatus, and theprobability increases that the access request signal is retransmitted.Meanwhile, when the threshold used in using sub-channel selectingsection 308 is made larger, an error rate of the access request signaldecreases in the base station apparatus, and therefore the probabilityis expected to decrease that the access request signal is retransmitted.However, the number of sub-channels decreases that can be selected inusing sub-channel selecting section 308, and therefore the probabilityof occurrence of collision of access request signals increases, and, asa result, it is considered that the probability may increase that theaccess request signal is retransmitted. Further, the probability thatthe access request signal is retransmitted is affected by an interferingamount and level of noise in the radio communication system. Therefore,in this Embodiment, with respect to the threshold used upon selection ofa sub-channel in using sub-channel selecting section 308, by selectingthe threshold so that the probability that the access request signal isretransmitted becomes the lowest, more specifically, by decreasing thethreshold in accordance with increases in the number of retransmissionsof the access request signal, it is possible to adapt to tradeoffexisting in adjusting the threshold and the interfering amount and levelof noise in the radio communication system.

Thus, according to communication terminal apparatus 300 according tothis Embodiment, in accordance with increases in the number ofretransmissions of the access request signal, the threshold used inselecting a sub-channel in using sub-channel selecting section 308 isadjusted to be smaller gradually, so that it is possible to decrease theprobability of occurrence of collision of access request signals withoutmaking the transmission power level of the access request signal higherthan necessary.

Hence, according to communication terminal apparatus 300 according tothis Embodiment, it is possible to decrease the probability ofoccurrence of collision of access request signals without making thetransmission power level of the access request signal higher thannecessary, so that, even when communication terminal apparatus 300 is amobile telephone and the like located near the cell edge, it is possibleto further prevent an access request signal transmitted fromcommunication terminal apparatus 300 from being an interfering signal inother adjacent cells and reducing throughput in the other cells.

In addition, in communication terminal apparatus 300 according to thisEmbodiment, the average reception power of the pilot signal decreases inaccordance with an increase in distance from the base station apparatus,and therefore, when the threshold used upon selection of a sub-channelin using sub-channel selecting section 308 is larger than the averagereception power, the number of sub-channels decreases that usingsub-channel selecting section 308 can select. Therefore, incommunication terminal apparatus 300 according to this Embodiment, thethreshold used upon selection of a sub-channel in using sub-channelselecting section 308 may be a relative value to the average receptionpower of the pilot signal.

Embodiment 3

FIG. 7 is a block diagram illustrating a configuration of communicationterminal apparatus 600 according to Embodiment 3 of the presentinvention. Communication terminal apparatus 600 has using sub-channelselecting section 608 instead of using sub-channel selecting section108, and has number-of-selection candidates setting section 632 betweenresponse determining section 107 and using sub-channel selecting section608 in communication terminal apparatus 100 according to Embodiment 1.

Based on the reception quality for each sub-channel reported fromreception quality measuring section 106, using sub-channel selectingsection 608 randomly selects one from a group of sub-channels withhigher reception quality where the number of sub-channels is within thesum of M and n reported from number-of-selection candidates settingsection 632, reports the selected sub-channel to sub-channel assigningsection 113, and further reports a transmission power level associatedwith the reception quality of the selected sub-channel to transmissionpower control section 114.

Number-of-selection candidates setting section 632 adds the number ofretransmissions n reported from response determining section 107 to anatural number M smaller than the total number of sub-channels in thereceived pilot signal, and reports the value of M+n to using sub-channelselecting section 608. In addition, M is assumed to be five (M=5) in thefollowing descriptions, and number-of-selection candidates settingsection 632 reports that M+n=5 when the number of retransmissions iszero to using sub-channel selecting section 608, and reports that M+n=6when the number of retransmissions is one.

The operation of communication terminal apparatus 600 will be describedbelow with reference to FIG. 8. FIG. 8 shows the reception SIR for eachsub-channel in the pilot signal measured in communication terminalapparatus 600. In addition, the reception SIR for each sub-channel asshown in FIG. 8 is the same as that in FIG. 3. Accordingly, upon firsttransmission of the access request signal, using sub-channel selectingsection 608 randomly selects one, for example, sub-channel #3, from fivesub-channels except sub-channel #2 of −4 dB that is the lowest receptionSIR, reports that the selected sub-channel is sub-channel #3 tosub-channel assigning section 113, and further reports a transmissionpower level associated with the reception SIR of sub-channel #3 totransmission power control section 114. Similarly, when the number ofretransmissions of the access request signal is one, using sub-channelselecting section 608 is capable of selecting randomly one from all ofsub-channels #1 to #6.

Thus, according to communication terminal apparatus 600, even when theaverage reception power of the pilot signal is low, a predeterminednumber of sub-channels that are candidates for selection in usingsub-channel selecting section 608 are reserved, so that it is possibleto effectively decrease the probability of occurrence of collision ofaccess request signals. Further, according to communication terminalapparatus 600 according to this Embodiment, a sub-channel to transmit orretransmit the access request signal is selected randomly fromsub-channels in descending order of the reception quality, and thereforethe transmission power is set to be low, so that it is possible toreduce interference imposed on adjacent other cells. By this means,according to communication terminal apparatus 600 according to thisEmbodiment, it is possible to effectively prevent reduction inthroughput in adjacent other cells.

Embodiment 4

FIG. 9 is a block diagram illustrating a configuration of communicationterminal apparatus 800 according to Embodiment 4 of the presentinvention. Communication terminal apparatus 800 has using sub-channelselecting section 808 instead of using sub-channel selecting section108, and has priority determining section 831 and number-of-selectioncandidates setting section 832 in communication terminal apparatus 100according to Embodiment 1.

Using sub-channel selecting section 808 first divides the receptionquality for each sub-channel reported from reception quality measuringsection 106 into a plurality of sub-channel groups, for example, a groupof sub-channels with low frequencies and a group of sub-channels withhigh frequencies. Further, using sub-channel selecting section 808selects the divided low-frequency sub-channel group or high-frequencysub-channel group based on a determination result on transmission databy priority determining section 831 reported from number-of-selectioncandidates setting section 832, described later. Furthermore, in theselected sub-channel group, based on the reception quality for eachsub-channel reported from reception quality measuring section 106, usingsub-channel selecting section 808 randomly selects one from the group ofsub-channels with higher reception quality where the number ofsub-channels is within the sum of M and n reported fromnumber-of-selection candidates setting section 832 described later,reports the selected sub-channel to sub-channel assigning section 113,and further reports a transmission power level associated with thereception quality of the selected sub-channel to transmission powercontrol section 114.

Priority determining section 831 determines the priority by judging atype of transmission data inputted from the control section and the likenot shown. More specifically, priority determining section 831determines that inputted transmission data is transmission data with ahigh priority when the transmission data is data having exacting delayrequirement, such as speech packet data, and determines that theinputted transmission data is transmission data with a low priority whenthe transmission data is Web data and the like, and reports thedetermination result to number-of-selection candidates setting section832.

Number-of-selection candidates setting section 832 reports thedetermination result on the priority of the transmission data reportedfrom priority determining section 831 to using sub-channel selectingsection 808, and based on the determination result, sets a naturalnumber M smaller than the total number of sub-channels of each of thehigh-frequency sub-channel group and the low-frequency sub-channel groupthat are divided by using sub-channel selecting section 808. Then, usingsub-channel selecting section 808 adds the number of retransmissions nreported from response determining section 107 to the set number M, andreports the value of M+n to sub-channel selecting section 808.

The operation of communication terminal apparatus 800 will be describedbelow with reference to FIG. 10. FIG. 10 shows the reception SIR foreach sub-channel in the pilot signal measured in communication terminalapparatus 800. In this Embodiment, as shown in FIG. 10, a pilot signalis configured with twelve sub-channels, and a sub-channel groupconfigured with six sub-channels with low frequencies is assigned tocommunication terminal apparatus 800 with a low priority of transmissiondata, and a sub-channel group configured with six sub-channels with highfrequencies is assigned to communication terminal apparatus 800 with ahigh priority of transmission data. In FIG. 10, with respect to thereception SIR for each sub-channel in the low-frequency sub-channelgroup, due to the effect of the frequency selective fading, sub-channel#1 is 6 dB, sub-channel #2 is −4 dB, sub-channel #3 is 10 dB,sub-channel #4 is 15 dB, sub-channel #5 is 2 dB, and sub-channel #6 is 5dB. Meanwhile, for the reception quality for each sub-channel in thehigh-frequency sub-channel group, sub-channel #1 is 8 dB, sub-channel #2is −3 dB, sub-channel #3 is 5 dB, sub-channel #4 is 18 dB, sub-channel#5 is 12 dB, and sub-channel #6 is 5 dB.

Then, in the case of the transmission data with the low priority asshown in FIG. 10(A), a mode is shown where number-of-selectioncandidates setting section 832 sets M at two (M=2) when the number ofretransmissions of the access request signal is zero, and usingsub-channel selecting section 808 selects sub-channel #3 of 10 dB orsub-channel #4 of 15 dB that are of two higher reception SIRs in thelow-frequency sub-channel group, and as a result, selects sub-channel#3. In the case of the transmission data with the high priority as shownin FIG. 10(B), another mode is shown where number-of-selectioncandidates setting section 832 sets M at four (M=4) when the number ofretransmissions of the access request signal is zero, and usingsub-channel selecting section 808 selects one of sub-channel #1 of 8 dB,sub-channel #3 of 5 dB, sub-channel #4 of 18 dB and sub-channel #5 of 12dB that are of four higher reception SIRs in the high-frequencysub-channel group, and as a result, selects sub-channel #1. In otherwords, as shown in FIG. 10, it does not happen that communicationterminal apparatuses 800 with different priorities of transmission datause the same sub-channel to transmit the access request signals.

Thus, according to communication terminal apparatus 800 according tothis Embodiment, using sub-channel selecting section 808 divides inadvance the reception quality for each sub-channel reported fromreception quality measuring section 106 into a high-frequencysub-channel group and a low-frequency sub-channel group, so that it ispossible to completely eliminate the risk that collision of accessrequest signals occurs among communication terminal apparatuses 800 withdifferent priorities of transmission data.

Further, according to communication terminal apparatus 800 according tothis Embodiment, number-of-selection candidates setting section 832 setsthe larger natural number M for transmission data with a high prioritythan the natural number M for transmission data with a low priority, andtherefore, communication terminal apparatus 800 holding transmissiondata with a higher priority has the lower probability of occurrence ofcollision of access request signals, and is thus capable of transmittingtransmission data held therein to the base station apparatus in a shorttime.

In addition, in this Embodiment, as shown in FIG. 10, that the case hasbeen described where a same number of sub-channels for use intransmission of the access request signals are assigned to communicationterminal apparatus 800 holding high-priority transmission data andcommunication terminal apparatus 800 holding low-priority transmissiondata, but the present invention is not limited to this case. Forexample, a larger number of sub-channels may be assigned tocommunication terminal apparatus 800 holding transmission data with ahigh priority.

Embodiment 5

In the above-mentioned Embodiments 1 to 4, the case has been describedas an example where an OFDM signal is transmitted or received in the TDDscheme. In Embodiment 5 of the present invention, the case will bedescribed as an example where an OFDM signal is transmitted or receivedin a frequency division duplex (FDD) scheme.

FIG. 11 is a block diagram illustrating a configuration of communicationterminal apparatus 1000 according to Embodiment 5 of the presentinvention. Communication terminal apparatus 1000 has reception qualitymeasuring section 1006 instead of reception quality measuring section106, has using sub-channel selecting section 1008 instead of usingsub-channel selecting section 108, and further has propagation pathcondition estimating section 1031 in communication terminal apparatus100 according to Embodiment 1.

Reception quality measuring section 1006 measures average receptionquality, for example, average SIR or average reception power level ofthe entire band—of all subcarriers in an OFDM signal—using a pilotsignal inputted from channel dividing section 103, and reports atransmission power level based on the measurement result to thetransmission power control section.

Using the pilot signal inputted from channel dividing section 103,propagation path condition estimating section 1031 estimates frequencyselectivity (speed of fading fluctuation in the frequency direction) ofthe propagation path, and speed of time fluctuation of the propagationpath. As a method of estimating the speed, estimation is made based onfluctuation amounts respectively in the frequency direction and timedirection of the pilot signal. Further, the frequency selectivity of thepropagation path can be estimated from delay spread of arrival signals,and the speed of time fluctuation of the propagation path can beestimated from moving speed of the communication terminal apparatus as amaximum Doppler frequency. In this Embodiment, delay spread and maximumDoppler frequency (fD) are used respectively for estimation of thefrequency selectivity of the propagation path and for estimation of thespeed of the time fluctuation.

When the number of retransmissions reported from response determiningsection 107 is zero, using sub-channel selecting section 1008 randomlyselects a sub-channel to be used and reports the selected sub-channel tosub-channel assigning section 113. When the number of retransmissionsreported from response determining section 107 is one or more, usingsub-channel selecting section 1008 randomly selects a sub-channel basedon the delay spread and maximum Doppler frequency reported frompropagation path condition estimating section 1031. For example, whenthe delay spread is less than or equal to a predetermined threshold (forexample, 100 ns) and the maximum Doppler frequency is less than or equalto a predetermined threshold (for example, 80 Hz), using sub-channelselecting section 1008 selects a sub-channel randomly from sub-channelsspaced apart from a sub-channel used in previous transmission by thepredetermined number of sub-channels (for example, five sub-channels) ormore in the frequency direction. Meanwhile, when the delay spread doesnot meet a value less than or equal to the predetermined threshold andthe maximum Doppler frequency does not meet a value less than or equalto the predetermined threshold, using sub-channel selecting section 1008selects a sub-channel randomly from sub-channels within a range lessthan the predetermined number of sub-channels in the frequencydirection. In other words, as the delay spread is smaller and themaximum Doppler frequency is lower, a sub-channel in frequency spacedfurther apart from the sub-channel used in previous transmission isselected.

Herein, it is assumed that a sub-channel is selected other than thesub-channel used in previous transmission. The selected sub-channel isreported to sub-channel assigning section 113.

The operation of communication terminal apparatus 1000 will be describedbelow with reference to FIG. 12.

In FIG. 12, it is assumed that communication terminal apparatuses 1000-1and 1000-2 both transmit access request signals concurrently to the samebase station apparatus, and that communication terminal apparatus 1000-1moves at a low speed and has small frequency selectively, andcommunication terminal apparatus 1000-2 moves at a high speed and haslarge frequency selectively.

In FIG. 12, upon first transmission of an access request signal, as theresult of random selection of sub-channel, each of communicationterminal apparatuses 1000-1 and 1000-2 selects the same sub-channel #10,and therefore both of first access request signals cannot be received inthe base station apparatus. As a result, both of communication terminalapparatuses 1000-1 and 1000-2 cannot receive an access permission signalwithin a response waiting period on the first access request signal, andretransmit access request signals.

Referring to FIG. 13, a method will be described where using sub-channelselecting section 1008 selects a sub-channel in retransmission of theaccess request signal. In FIG. 13, in step (hereinafter, referred to as“ST”) 1201, it is determined whether or not the maximum Dopplerfrequency (fD) exceeds a threshold (80 Hz), and the processing shifts toST1203 when the maximum Doppler frequency is determined to exceed thethreshold (Yes), and the processing shifts to ST1202 when the maximumDoppler frequency is determined not to exceed the threshold (No).

In ST1202, it is determined whether or not the delay spread exceeds athreshold (100 ns), and the processing shifts to ST1203 when the delayspread is determined to exceed the threshold (Yes), and the processingshifts to ST1204 when the delay spread is determined not to exceed thethreshold (No).

In ST1203, a sub-channel is randomly selected from sub-channels within arange less than the predetermined number P of sub-channels from thesub-channel used in the previous transmission. In ST1204, a sub-channelis randomly selected from sub-channels spaced apart from the sub-channelused in the previous transmission by the predetermined number ofsub-channels P or more.

Referring again to FIG. 12, communication terminal apparatus 1000-1 hassmall frequency selectivity (delay spread=30 ns) and a low moving speed(fD=5 Hz), and meets the conditions that the delay spread is less thanor equal to the threshold (100 ns) and that the maximum Dopplerfrequency is less than or equal to the threshold (80 Hz). Therefore, inretransmission, based on the method of selecting a sub-channel as shownin FIG. 13, communication terminal apparatus 1000-1 selects asub-channel randomly from sub-channels spaced apart from sub-channel #10used in the previous transmission by frequencies of five sub-channels ormore, and as a result, selects sub-channel #3 to transmit. Meanwhile,communication terminal apparatus 1000-2 has large frequency selectivity(delay spread=200 ns) and a high moving speed (fD=150 Hz), and does notmeet the conditions that the delay spread is less than or equal to 100ns and that the maximum Doppler frequency is less than or equal to 80Hz. Therefore, communication terminal apparatus 1000-2 selects asub-channel randomly from sub-channels within a range less than fivesub-channels from sub-channel #10 used in the previous transmission, andas a result, selects sub-channel #8 to transmit.

Described next is the reason why a sub-channel in frequency spacedfurther apart from the sub-channel used in the previous transmission isselected as the delay spread is smaller and the maximum Dopplerfrequency is lower. As the cause of RACH transmission failure, a drop inreception power on the frequency axis caused by frequency selectivefading and collision (where communication terminals of two or more userstransmit signals concurrently on the same sub-channel) can beconsidered.

Upon retransmission on the RACH, it is possible to decrease theprobability of transmission failure due to the above-mentioned twocauses, and to reduce the transmission delay. For the drop in receptionpower due to frequency selective fading, by performing transmissionusing a sub-channel of a propagation path different from the propagationpath of the sub-channel previously used when transmission fails, it ispossible to decrease the probability that the reception power drops dueto frequency selective fading also upon retransmission. Therefore, it isonly necessary to use a sub-channel spaced sufficiently apart from thepreviously used sub-channel on the frequency axis.

However, when selection is limited to the sufficiently spacedsub-channels on the frequency axis, the probability increases thatcollision occurs upon retransmission. Hence, in order to decrease theprobability of collision upon retransmission, based on the delay spreadand maximum Doppler frequency, a distance is determined by which asub-channel is spaced apart from the previously used sub-channel on thefrequency axis. More specifically, in a user with a low maximum Dopplerfrequency and small delay spread, a variation amount of the propagationpath is small between retransmission and previous transmission andfluctuation in the propagation path in the frequency direction ismoderate, and therefore a sub-channel spaced apart from the previouslyused sub-channel by a predetermined number of sub-channels or more onthe frequency axis is selected. In users other than the above-mentioneduser, the propagation path varies with time or substantially fluctuatesin the frequency direction, and therefore a possibility is high that thepropagation path is different even when the sub-channel is not spaced bya large amount on the frequency axis. Accordingly, a sub-channel isselected from a predetermined range from the previously usedsub-channel. Generally, in a cellular system, users with variouspropagation path conditions exist it the cell, so that, by selecting adifferent sub-channel for each user by the above-mentioned selectionmethod, it is possible to improve the probability of avoiding collision.

Thus, according to communication terminal apparatus 1000 according tothis Embodiment, a subcarrier used upon retransmission on the RACH isspaced apart from the previously used subcarrier on the frequency axisaccording to a fluctuation amount of the propagation path from previoustransmission, so that it is possible to decrease the probability oftransmission failure on the RACH due to collision and drop of fading. Asa result, it is not necessary to provide a long back-off time, and thenumber of retransmissions on the RACH can be decreased, so that it ispossible to start communication in a short time, and improve thecommunication quality in packet communication having exacting delayrequirement, such as speech communication and video transmission.Further, by decreasing the number of retransmissions, it is possible tosuppress reduction in throughput.

In addition, this Embodiment describes the case of using both thefrequency selectivity and speed of time fluctuation as propagation pathconditions, but only one of the both may be used. Further, the frequencyselectively may be defined by coherent bandwidth (bandwidth where thepropagation path can be regarded as being constant) or the like, and thespeed of time fluctuation may be defined by coherent time (time wherethe propagation path can be regarded as being constant) or the like.

Further, a plurality of thresholds may be set on each of delay spreadand maximum Doppler frequency, and a selection width of a plurality ofsub-channels corresponding to the thresholds may be set.

Furthermore, the case has been described where response determiningsection 107 counts the number of retransmissions of the access requestsignal, but response determining section 107 may determine only whetheror not to retransmit the access request signal.

Embodiment 6

Embodiment 6 of the present invention describes the case where aplurality of sub-channels explained in Embodiment 5 is grouped.

FIG. 14 is a block diagram illustrating a configuration of communicationterminal apparatus 1300 according to Embodiment 6 of the presentinvention. Communication terminal apparatus 1300 has using sub-channelselecting section 1308 instead of using sub-channel selecting section1008 and further has group selecting section 1331 in communicationterminal apparatus 1000 according to Embodiment 5.

Group selecting section 1331 selects a single group from groups eachconfigured with a plurality of sub-channels. More specifically, when thenumber of retransmissions reported from response determining section 107is zero, group selecting section 1308 randomly selects a group to beused. When the number of retransmissions reported from responsedetermining section 107 is one or more, group selecting section 1331randomly selects a group based on the maximum Doppler frequency anddelay spread reported from propagation path condition estimating section1031. For example, when the maximum Doppler frequency is more than orequal to a predetermined threshold (for example, 80 Hz), group selectingsection 1331 selects the same group as the group used in the previoustransmission. When the maximum Doppler frequency is less than thethreshold, group selecting section 1308 selects a group different fromthe previously used group. Further, in the case where the maximumDoppler frequency is less than the predetermined threshold, groupselecting section 1331 selects a group within a range less than apredetermined number of groups (for example, two groups) from the groupused in the previous transmission when the delay spread is more than orequal to a predetermined threshold (100 ns), and selects a group spacedapart from the group used in the previous transmission by thepredetermined number of groups or more when the delay spread is lessthan the predetermined threshold. The selected group is reported tousing sub-channel selecting section 1308.

Using sub-channel selecting section 1308 selects a single sub-channelrandomly from sub-channels of the group reported from group selectingsection 1331, and reports the selected sub-channel to sub-channelassigning section 113.

The operation of communication terminal apparatus 1300 will be describedbelow with reference to FIG. 15.

In FIG. 15, it is assumed that sixteen sub-channels are divided intofour groups each configured with four sub-channels, and thatcommunication terminal apparatuses 1300-1 and 1300-2 first transmitaccess request signals to the same base station on the same conditionsas in FIG. 12. In other words, neither of communication terminalapparatuses 1300-1 and 1300-2 can receive an access permission signalwithin a predetermined waiting period on the first access requestsignals, and the access request signals are retransmitted.

In retransmission of the access request signals, a method of selecting asub-channel will be described below with reference to FIG. 16. In FIG.16, in ST1501, group selecting section 1331 determines whether or notthe maximum Doppler frequency (fD) exceeds the threshold (80 Hz), andthe processing shifts to ST1502 when the maximum Doppler frequency isdetermined to exceed the threshold (Yes), and the processing shifts toST1503 when the maximum Doppler frequency is determined not to exceedthe threshold (No).

In ST1502, group selecting section 1331 selects a group different fromanother group used in the previous transmission, and the processingshifts to ST1506.

In ST1503, group selecting section 1331 determines whether or not thedelay spread exceeds the threshold (100 ns), and the processing shiftsto ST1504 when the delay spread is determined to exceed the threshold(Yes), and the processing shifts to ST1505 when the delay spread isdetermined not to exceed the threshold (No).

In ST1504, group selecting section 1331 selects a group randomly fromgroups within a range less than the predetermined number of groups Nfrom the group used in the previous transmission. In ST1505, groupselecting section 1331 selects a group randomly from groups spaced apartfrom the group used in the previous transmission by the predeterminednumber of groups N or more.

In ST1506, using sub-channel selecting section 1308 selects asub-channel randomly from the selected group.

Referring to FIG. 15 again, upon retransmission, communication terminalapparatus 1300-1 has the maximum Doppler frequency of 5 Hz that is lessthan the predetermined threshold (for example, 80 Hz), and therefore,based on the sub-channel selecting method as shown in FIG. 16, selects agroup different from the group (group C) used in the previoustransmission. Further, since the delay spread is 30 ns and less than thepredetermined threshold (for example, 100 ns), communication terminalapparatus 1300-1 selects group A spaced apart from group C by two groupsor more. Then, as the result of selecting a sub-channel randomly fromgroup A, communication terminal apparatus 1300-1 performs transmissionusing sub-channel #3.

Meanwhile, communication terminal apparatus 1300-2 has the maximumDoppler frequency of 150 Hz that is more than the predeterminedthreshold (for example, 80 Hz), and therefore, selects group C as thesame group as the group used in the previous transmission. Then, as theresult of selecting a sub-channel randomly from group C, communicationterminal apparatus 1300-2 performs transmission using sub-channel #12.

Thus, according to communication terminal apparatus 1300 according tothis Embodiment, upon retransmission on the RACH, a sub-channel isselected from the same group as in the previous transmission when afluctuation amount in the time direction of the propagation path islarge and the maximum Doppler frequency is high in each communicationterminal apparatus, and a sub-channel is selected from a group differentfrom that in the previous transmission when a fluctuation amount in thetime direction of the propagation path is small and the maximum Dopplerfrequency is low, so that it is possible to reduce the probability ofRACH transmission failure due to a drop in fading and collision.Further, upon selection of a group different from that in the previoustransmission, by selecting a group in frequency spaced apart from thegroup used in the previous transmission according to the fluctuationamount in the frequency direction of the propagation path—delay spread—,it is possible to further reduce the probability of RACH transmissionfailure due to a drop in fading. As a result, it is not necessary toprovide a long back-off time, and the number of retransmissions on theRACH can be decreased, so that it is possible to start communication ina short time and improve the communication quality in packetcommunication having exacting delay requirement, such as speechcommunication and video transmission. Further, by decreasing the numberof retransmissions, it is possible to suppress reduction in throughput.

In addition, in this Embodiment, the case has been described where boththe frequency selectivity and speed of the time fluctuation are used aspropagation path conditions, but only one of the both may be used.Further, the frequency selectively may be defined by coherent bandwidth(bandwidth where the propagation path can be regarded as being constant)or the like, and the speed of time fluctuation may be defined bycoherent time (time where the propagation path can be regarded as beingconstant) or the like.

Further, the case has been described as an example where sub-channels ina group are OFDM subcarriers, but the present invention is not limitedthereto. For example, the sub-channels may be spreading codes orpatterns of symbol repetition, for example.

Each of Embodiments of the present invention has been described in theforegoing.

In addition, in each of the above-mentioned Embodiments, the case hasbeen described where the communication terminal apparatus handles as oneunit a carrier group configured with a plurality of subcarriers of anOFDM signal, measures the reception quality of a pilot signal on a basisof the carrier group—on a sub-channel basis—, and transmits an accessrequest signal to the base station apparatus, but the present inventionis not limited to this case. For example, the communication terminalapparatus may measure the reception quality of a pilot signal on asubcarrier basis in an OFDM signal, and transmit an access requestsignal to the base station apparatus using one of subcarriers.

The present invention can be applied to other signals other than theaccess request signals, if the signals are data transmitted using theRACH. Further, the RACH may be expressed by a contention channel,competition channel, or the like.

Further, in each of the above-mentioned Embodiments, the case has beendescribed as an example where OFDM is used as a transmission scheme, butthe present invention is not limited to the case. For example, thepresent invention can be applied to a transmission scheme such as FDMA(Frequency Division Multiple Access) for transmitting a single-carriersignal using a plurality of carriers or frequencies, and the sameeffects can be obtained. In this case, the sub-channel indicates onesingle-carrier signal, for example.

Furthermore, the present invention can be applied to a transmissionscheme such as IFDMA (Interleaved FDMA) using a frequency arrangementwith dispersion in comb-shape, that is, equal intervals. In this case,the sub-channel is one IFDMA signal. The IFDMA signal may be referred toas distributed channel.

Still furthermore, the channel may be divided further by spreading codeand the like in one single-carrier signal or IFDMA signal. In this case,the sub-channel indicates a spreading code in one single-carrier signal.

Moreover, different transmission schemes may be used between uplink anddownlink.

Further, in each of the above-mentioned Embodiments, the presentinvention can be similarly applied to a case of transmitting a preambleprior to transmission on the RACH.

Furthermore, in each of the above-mentioned Embodiments, one of the TDDscheme and FDD scheme has been assumed and described, but the presentinvention can be applied to both of the schemes.

Furthermore, although a case has been described as an example in whichthe present invention is implemented with hardware, the presentinvention can be implemented with software.

Still further, each function block used to explain the above-describedembodiments is typically implemented as an LSI constituted by anintegrated circuit. These may be individual chips or may partially ortotally contained on a single chip. Here, each function block isdescribed as an LSI, but this may also be referred to as “IC”, “systemLSI”, “super LSI”, “ultra LSI” depending on differing extents ofintegration.

Further, the method of circuit integration is not limited to LSI's, andimplementation using dedicated circuitry or general purpose processorsis also possible. After LSI manufacture, utilization of a programmableFPGA (Field Programmable Gate Array) or a reconfigurable processor inwhich connections and settings of circuit cells within an LSI can bereconfigured is also possible.

Further, if integrated circuit technology comes out to replace LSI's asa result of the development of semiconductor technology or a derivativeother technology, it is naturally also possible to carry out functionblock integration using this technology. Application in biotechnology isalso possible.

A first aspect of the present invention is a communication terminalapparatus provided with: a sub-channel selector that selects asub-channel to be used in transmission of a random access signal from agroup of sub-channels defined by a class of frequency, and selects thesub-channel different from previously used sub-channel wheneverretransmitting the random access signal; and a transmitter thattransmits the random access signal using the selected sub-channel.

A second aspect of the present invention is a communication terminalapparatus further having: a receiver that receives a pilot signal; and ameasurer that measures reception quality of the received pilot signalfor each sub-channel defined by a class of frequency in theabove-mentioned aspect, wherein the sub-channel selector selects asub-channel from a group of sub-channels using the measured receptionquality.

A third aspect of the present invention is a communication terminalapparatus in the above-mentioned aspect wherein the sub-channel selectorrandomly selects one from a group of sub-channels with the receptionquality measured in the measurer being more than or equal to apredetermined threshold.

A fourth aspect of the present invention is a communication terminalapparatus in the above-mentioned aspect wherein the sub-channel selectorselects a sub-channel with the reception quality measured in themeasurer being the highest.

A fifth aspect of the present invention is a communication terminalapparatus in the above-mentioned aspect wherein the sub-channel selectorrandomly selects one from M sub-channels with the reception qualitymeasured in the measurer being higher.

A sixth aspect of the present invention is a communication terminalapparatus further having a response determiner that determines thepresence or absence of a response of the transmitted access requestsignal, and thereby counts the number of retransmissions of the randomaccess signal in the above-mentioned aspect, wherein the sub-channelselector decreases the threshold used upon selecting of the sub-channelin accordance with an increase in the number of retransmissions countedin the response determiner.

A seventh aspect of the present invention is a communication terminalapparatus further having a response determiner that determines thepresence or absence of a response of the transmitted random accesssignal, and thereby counts the number of retransmissions of the accessrequest signal in the above-mentioned aspect, wherein the sub-channelselector selects a sub-channel with the reception quality measured inthe measurer being the highest when the number of retransmissionscounted in the response determiner is zero, and selects a sub-channelwith the lower reception quality measured in the measurer correspondingto the counted number of retransmissions from the highest receptionquality when the number of retransmissions counted in the responsedeterminer is one or more.

An eighth aspect of the present invention is a communication terminalapparatus further having a response determiner that determines thepresence or absence of a response of the transmitted random accesssignal, and thereby counts the number of retransmissions of the accessrequest signal in the above-mentioned aspect, wherein the sub-channelselector selects a sub-channel with the reception quality measured inthe measurer being the highest when the number of retransmissionscounted in the response determiner is zero, and randomly selects onefrom M sub-channels with the reception quality measured in the measurerbeing higher when the number of retransmissions counted in the responsedeterminer is one or more.

A ninth aspect of the present invention is a communication terminalapparatus in the above-mentioned aspect wherein the sub-channel selectorincreases the number M of sub-channels with higher reception quality inaccordance with an increase in the counted number of retransmissionswhen the number of retransmissions counted in the response determiner isone or more.

A tenth aspect of the present invention is a communication terminalapparatus further having a priority determiner that determines apriority of transmission data to be transmitted after communicationstarts by transmission of the random access signal in theabove-mentioned aspect, wherein the sub-channel selector increases thenumber M of sub-channels with higher reception quality according to thepriority of the transmission data determined in the priority determiner.

An eleventh aspect of the present invention is a communication terminalapparatus further having a response determiner that determines thepresence or absence of a response of the transmitted random accesssignal, and thereby counts the number of retransmissions of the accessrequest signal in the above-mentioned aspect, wherein the sub-channelselector increases the number M of sub-channels with higher receptionquality in accordance with an increase in the counted number ofretransmissions when the number of retransmissions counted in theresponse determiner is one or more.

A twelfth aspect of the present invention is a communication terminalapparatus further having in the above-mentioned aspect: a responsedeterminer that determines the presence or absence of a response of thetransmitted random access signal, and thereby determines whether or notto retransmit the random access signal; and an estimator that estimatesa propagation path condition, wherein the sub-channel selector selects asub-channel to be used in transmission of the random access signal basedon the propagation path condition estimated in the estimator, when theresponse determiner determines to retransmit the random access signal.

A thirteenth aspect of the present invention is a communication terminalapparatus in the above-mentioned aspect wherein the estimator estimatesthe propagation path condition as speed of fluctuation in the timedirection of the propagation path.

A fourteenth aspect of the present invention is a communication terminalapparatus in the above-mentioned aspect wherein the sub-channel selectorselects a sub-channel with a difference in frequency from a sub-channelused in previous transmission of the random access signal being lessthan a predetermined amount when the speed of the fluctuation in thetime direction of the propagation path is more than or equal to apredetermined threshold, and selects a sub-channel with the differencein frequency from the sub-channel used in the previous transmission ofthe random access signal being more than or equal to the predeterminedamount when the speed is less than the predetermined threshold.

A fifteenth aspect of the present invention is a communication terminalapparatus in the above-mentioned aspect wherein the estimator estimatesthe propagation path condition as speed of fluctuation in the frequencydirection of the propagation path.

A sixteenth aspect of the present invention is a communication terminalapparatus in the above-mentioned aspect wherein the sub-channel selectorselects a sub-channel with a difference in frequency from a sub-channelused in previous transmission of the random access signal is less than apredetermined amount when the speed of the fluctuation in the frequencydirection of the propagation path is more than or equal to apredetermined threshold, and selects a sub-channel with the differencein frequency from the sub-channel used in the previous transmission ofthe random access signal is more than or equal to the predeterminedamount when the speed is less than the predetermined threshold.

A seventeenth aspect of the present invention is a communicationterminal apparatus in the above-mentioned aspect wherein the estimatorestimates the propagation path condition as both the speed of thefluctuation in the time direction and the speed of the fluctuation inthe frequency direction of the propagation path.

An eighteenth aspect of the present invention is a communicationterminal apparatus in the above-mentioned aspect wherein the sub-channelselector selects a sub-channel with a difference in frequency from asub-channel used in previous transmission of the random access signalbeing more than or equal to the predetermined amount when the speed ofthe fluctuation in the time direction of the propagation path is lessthan a predetermined first threshold and the speed of the fluctuation inthe frequency direction is less than a predetermined second threshold,and selects a sub-channel with the difference in frequency from thesub-channel used in the previous transmission of the random accesssignal being less than the predetermined amount when the speed of thefluctuation in the time direction of the propagation path is more thanor equal to the predetermined first threshold or the speed of thefluctuation in the frequency direction is more than or equal to thepredetermined second threshold.

A nineteenth aspect of the present invention is a communication terminalapparatus in the above-mentioned aspect wherein the sub-channel selectorselects one of groups each configured with a plurality of sub-channelsbased on the propagation path condition, and further selects one ofsub-channels of the selected group.

A twentieth aspect of the present invention is a communication terminalapparatus in the above-mentioned aspect wherein the sub-channel selectorselects the same group as a group selected in previous transmission ofthe random access signal when the speed of the fluctuation in the timedirection of the propagation path condition is more than or equal to apredetermined threshold, and selects a group different from the groupselected in the previous transmission of the random access signal whenthe speed is less than the predetermined threshold.

A twenty-first aspect of the present invention is a communicationterminal apparatus in the above-mentioned aspect wherein the sub-channelselector selects a group with a difference in frequency from the groupselected in previous transmission of the random access signal being lessthan a predetermined amount when the speed of the fluctuation in thetime direction of the propagation path is less than a predeterminedfirst threshold and the speed of the fluctuation in the frequencydirection of the propagation path is more than or equal to apredetermined second threshold, and selects a group with the differencein frequency from the group selected in the previous transmission of therandom access signal being more than or equal to the predeterminedamount when the speed of, the fluctuation in the frequency direction ofthe propagation path is less than the predetermined second threshold.

A twenty-second aspect of the present invention is a communicationterminal apparatus in the above-mentioned aspect wherein the sub-channelis a subcarrier or a subcarrier block in an OFDM signal.

A twenty-third aspect of the present invention is a communicationterminal apparatus in the above-mentioned aspect wherein the sub-channelis one single-carrier signal in an FDMA system.

A twenty-fourth aspect of the present invention is a radio communicationmethod having: a sub-channel selecting step of selecting a sub-channelto be used in transmission of a random access signal from a group ofsub-channels defined by a class of frequency, and selecting thesub-channel different from previously used sub-channel wheneverretransmitting the random access signal; and a transmitting step oftransmitting the random access signal using the selected sub-channel.

The present application is based on Japanese Patent Applications No.2004-207196 filed on Jul. 14, 2004, and No. 2005-200276 filed on Jul. 8,2005, entire contents of which are expressly incorporated by referenceherein.

INDUSTRIAL APPLICABILITY

The communication terminal apparatus and radio communication methodaccording to the present invention provide an advantage of decreasingthe probability of occurrence of collision of access request signals andreducing the number of retransmissions of the access request signal, andare useful as a mobile radio communication terminal apparatus and thelike such as a mobile telephone and PDA.

1. A communication terminal apparatus comprising: a sub-channel selectorthat selects a sub-channel to be used in transmission of a random accesssignal from a group of sub-channels defined by a class of frequency, andselects the sub-channel different from previously used sub-channelwhenever retransmitting the random access signal; and a transmitter thattransmits the random access signal using the selected sub-channel. 2.The communication terminal apparatus according to claim 1, furthercomprising: a receiver that receives a pilot signal; and a measurer thatmeasures reception quality of the received pilot signal for eachsub-channel defined by a class of frequency, wherein the sub-channelselector selects a sub-channel from a group of sub-channels using themeasured reception quality.
 3. The communication terminal apparatusaccording to claim 2, wherein the sub-channel selector randomly selectsone from a group of sub-channels with the reception quality measured inthe measurer being more than or equal to a predetermined threshold. 4.The communication terminal apparatus according to claim 2, wherein thesub-channel selector selects a sub-channel with the reception qualitymeasured in the measurer being the highest.
 5. The communicationterminal apparatus according to claim 2, wherein the sub-channelselector randomly selects one from M sub-channels with the receptionquality measured in the measurer being higher.
 6. The communicationterminal apparatus according to claim 3, further comprising a responsedeterminer that determines the presence or absence of a response of thetransmitted random access signal, and thereby counts the number ofretransmissions of the random access signal, wherein the sub-channelselector decreases the threshold used upon selecting of the sub-channelin accordance with an increase in the number of retransmissions countedin the response determiner.
 7. The communication terminal apparatusaccording to claim 2, further comprising a response determiner thatdetermines the presence or absence of a response of the transmittedrandom access signal, and thereby counts the number of retransmissionsof the random access signal, wherein the sub-channel selector selects asub-channel with the reception quality measured in the measurer beingthe highest when the number of retransmissions counted in the responsedeterminer is zero, and selects a sub-channel with the lower receptionquality measured in the measurer corresponding to the counted number ofretransmissions from the highest reception quality when the number ofretransmissions counted in the response determiner is one or more. 8.The communication terminal apparatus according to claim 2, furthercomprising a response determiner that determines the presence or absenceof a response of the transmitted random access signal, and therebycounts the number of retransmissions of the random access signal,wherein the sub-channel selector selects a sub-channel with thereception quality measured in the measurer being the highest when thenumber of retransmissions counted in the response determiner is zero,and randomly selects one from M sub-channels with the reception qualitymeasured in the measurer being higher when the number of retransmissionscounted in the response determiner is one or more.
 9. The communicationterminal apparatus according to claim 8, wherein the sub-channelselector increases the number M of sub-channels with higher receptionquality in accordance with an increase in the counted number ofretransmissions when the number of retransmissions counted in theresponse determiner is one or more.
 10. The communication terminalapparatus according to claim 5, further comprising a priority determinerthat determines a priority of transmission data to be transmitted aftercommunication starts by transmission of the random access signal,wherein the sub-channel selector increases the number M of sub-channelswith higher reception quality according to the priority of thetransmission data determined in the priority determiner.
 11. Thecommunication terminal apparatus according to claim 10, furthercomprising a response determiner that determines the presence or absenceof a response of the transmitted random access signal, and therebycounts the number of retransmissions of the random access signal,wherein the sub-channel selector increases the number M of sub-channelswith higher reception quality in accordance with an increase in thecounted number of retransmissions when the number of retransmissionscounted in the response determiner is one or more.
 12. The communicationterminal apparatus according to claim 1, further comprising: a responsedeterminer that determines the presence or absence of a response of thetransmitted random access signal, and thereby determines whether or notto retransmit the random access signal; and an estimator that estimatesa propagation path condition, wherein the sub-channel selector selects asub-channel to be used in transmission of the random access signal basedon the propagation path condition estimated in the estimator, when theresponse determiner determines to retransmit the random access signal.13. The communication terminal apparatus according to claim 12, whereinthe estimator estimates the propagation path condition as speed offluctuation in the time direction of the propagation path.
 14. Thecommunication terminal apparatus according to claim 13, wherein thesub-channel selector selects a sub-channel with a difference infrequency from a sub-channel used in previous transmission of the randomaccess signal being less than a predetermined amount when the speed ofthe fluctuation in the time direction of the propagation path is morethan or equal to a predetermined threshold, and selects a sub-channelwith the difference in frequency from the sub-channel used in theprevious transmission of the random access signal being more than orequal to the predetermined amount when the speed is less than thepredetermined threshold.
 15. The communication terminal apparatusaccording to claim 12, wherein the estimator estimates the propagationpath condition as speed of fluctuation in the frequency direction of thepropagation path.
 16. The communication terminal apparatus according toclaim 15, wherein the sub-channel selector selects a sub-channel with adifference in frequency from a sub-channel used in previous transmissionof the random access signal being less than a predetermined amount whenthe speed of the fluctuation in the frequency direction of thepropagation path is more than or equal to a predetermined threshold, andselects a sub-channel with the difference in frequency from thesub-channel used in the previous transmission of the random accesssignal being more than or equal to the predetermined amount when thespeed is less than the predetermined threshold.
 17. The communicationterminal apparatus according to claim 12, wherein the estimatorestimates the propagation path condition as both speed of fluctuation inthe time direction and speed of fluctuation in the frequency directionof the propagation path.
 18. The communication terminal apparatusaccording to claim 17, wherein the sub-channel selector selects asub-channel with a difference in frequency from a sub-channel used inprevious transmission of the random access signal being more than orequal to a predetermined amount when the speed of the fluctuation in thetime direction of the propagation path is less than a predeterminedfirst threshold and the speed of the fluctuation in the frequencydirection is less than a predetermined second threshold, and selects asub-channel with the difference in frequency from the sub-channel usedin the previous transmission of the random access signal being less thanthe predetermined amount when the speed of the fluctuation in the timedirection of the propagation path is more than or equal to thepredetermined first threshold or the speed of the fluctuation in thefrequency direction is more than or equal to the predetermined secondthreshold.
 19. The communication terminal apparatus according to claim12, wherein the sub-channel selector selects one of groups eachconfigured with a plurality of sub-channels based on the propagationpath condition, and further selects one of sub-channels of the selectedgroup.
 20. The communication terminal apparatus according to claim 19,wherein the sub-channel selector selects the same group as a groupselected in previous transmission of the random access signal when thespeed of the fluctuation in the time direction of the propagation pathcondition is more than or equal to a predetermined threshold, andselects a group different from the group selected in the previoustransmission of the random access signal when the speed is less than thepredetermined threshold.
 21. The communication terminal apparatusaccording to claim 20, wherein the sub-channel selector selects a groupwith a difference in frequency from the group selected in previoustransmission of the random access signal being less than a predeterminedamount when the speed of the fluctuation in the time direction of thepropagation path is less than a predetermined first threshold and speedof fluctuation in the frequency direction of the propagation path ismore than or equal to a predetermined second threshold, and selects agroup with the difference in frequency from the group selected in theprevious transmission of the random access signal being more than orequal to the predetermined amount when the speed of the fluctuation inthe frequency direction of the propagation path is less than thepredetermined second threshold.
 22. The communication terminal apparatusaccording to claim 1, wherein the sub-channel is a subcarrier or asubcarrier block in an OFDM signal.
 23. The communication terminalapparatus according to claim 1, wherein the sub-channel is onesingle-carrier signal in an FDMA system.
 24. A radio communicationmethod comprising: a sub-channel selecting step of selecting asub-channel to be used in transmission of a random access signal from agroup of sub-channels defined by a class of frequency, and selecting thesub-channel different from previously used sub-channel wheneverretransmitting the random access signal; and a transmitting step oftransmitting the random access signal using the selected sub-channel.